TRANSLOCATION OF NUTRITIVE MATERIALS 213 
cells, so that very soon the whole of the parenchyma of the 
constructive region is plentifully supplied with the sugar. 
This parenchyma abuts, however, on other cells which con- 
tain no chloroplasts, especially the sheaths and the bast of 
the fibro-vascular bundles. Diffusion of sugar into these 
takes place, and proceeds from cell to cell, especially among 
the delicate bast tissue, so that a stream of sugar is soon dif- 
fusing all along the bast. As a rule it does not penetrate 
very far beyond this tissue, owing largely to the anatomical 
arrangements of the parts and the great facility which the 
structure of the bast affords for this diffusion. So long as 
the manufacture goes on, therefore, there is an outflow of 
the manufactured carbohydrates from the region of its forma- 
tion, the ultimate and even the temporary direction of the 
stream being determined by other factors which we shall 
consider later. 
This removal of sugar from the leaf can be proved by 
several observations. We find but little of it in the meso- 
phyll of the leaf, though we know it is being continually 
produced there. We find it fairly easily in the bast of the 
veins, and if a leaf is cut off from the stem while construc- 
tion is going on, so that it cannot be transported away, it 
can very soon be detected in the mesophyll cells as well. 
This, however, is not all. The process of diffusion is a 
slow one and does not serve to remove the sugar as fast 
as it is formed. The excessive formation of sugar would 
goon lead to such a saturation of the sap as would at any 
rate temporarily inhibit its construction, were it not for 
another agency at work. The chloroplasts are endowed. 
with another property than that so far described, which is 
now called into play. This is a peculiarity of the body of 
the plastid, and is quite independent of the colouring 
matter, being shared by other quite colourless plastids 
which occur in other parts of the plant. These structures 
have the power of converting sugar into starch, a power 
which we must examine more fully in a subsequent chapter. 
The transformation is apparently a process of secretion. 
